Clock generator

ABSTRACT

A clock generator receives first and second clock signals, and input representing a desired frequency ratio. A comparison is made between frequencies of an output clock signal and the first clock signal, and a first error signal represents the difference between the desired frequency ratio and this comparison result. The first error signal is filtered. A comparison is made between frequencies of the output clock signal and the second clock signal, and a second error signal represents the difference between the filtered first error signal and this comparison result. The second error signal is filtered. A numerically controlled oscillator receives the filtered second error signal and generates an output clock signal. As a result, the output clock signal has the jitter characteristics of the first input clock signal over a useful range of jitter frequencies and the frequency accuracy of the second input clock signal.

This application claims the benefit of U.S. Provisional Application No.61/562,327, filed on Nov. 21, 2012, the disclosure of which isincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a frequency locked loop, and in particular toa frequency locked loop suitable for use as a clock generator inintegrated circuit form.

2. Description of the Related Art

It is known to use a frequency locked loop (FLL) to generate a clocksignal at a frequency that is a multiple of the frequency of an existing(input) clock signal. For example, a high-frequency output clock clocksa counter. At edges of the existing clock signal, the accumulated countis latched and the counter is reset. Thus, the count represents theratio of the frequency of the output clock to the frequency of theexisting clock. This ratio is subtracted from an input valuerepresenting the desired ratio, and the resulting frequency error signalis fed to a filter. The filter integrates the frequency error to producean integrated error signal, which is used to drive a numericallycontrolled oscillator, with the output of the numerically controlledoscillator being taken as the high-frequency output clock fed back toclock the counter. The feedback of the output clock means that, if itsfrequency becomes higher than the desired frequency, a negativefrequency error signal is generated, causing the output frequency to bereduced. Conversely, if the frequency of the output clock becomes lowerthan the desired frequency, a positive frequency error signal isgenerated, causing the output frequency to increase. Thus the frequencyof the generated clock converges to the desired frequency.

One application of such FLLs is in digital audio signal processing orreproduction circuitry, and in host devices employing such circuitry,including but not limited to, portable electronic devices, mobilephones, PDA's, netbooks, laptops, tablets, computers. For high qualityaudio reproduction it is important that the clock driving an outputdigital-to-analog converter has low jitter, especially in the audiofrequency band, to avoid noise, distortion or spurious tones. The clockaccompanying the input data may not be high quality, due to either apoor quality clock source or degradation along the transmission channel,and the clock for signal processing may need to be a multiple of thedata transmission clock.

Also the generated clock must be closely synchronised to the incomingdata: any accumulated clock slippage may result in dropped samples orgaps in the data to be processed.

Further, in some applications the clocks may be intermittent, perhapsdue to the data being transmitted in bursts. Or the clock source maychange as the modality of the host device is changed to servicedifferent use scenarios and to save any unnecessary power consumption.Yet any such clock transition should be undetectable in the reproducedaudio.

Also preferably, for economic implementation in integrated circuit form,there should be minimal external components such as large capacitors: apredominantly digital solution is desirable.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda clock generator, for generating an output clock signal, the clockgenerator comprising:

a first clock signal input, for receiving a first input clock signal;

a first frequency comparator, for generating a first frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the first input clock signal;

a first subtractor, for forming a first error signal representing adifference between an input desired frequency ratio and the firstfrequency comparison signal;

a first digital filter, for receiving the first error signal and forminga filtered first error signal;

a second clock signal input, for receiving a second input clock signal;

a second frequency comparator, for generating a second frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the second input clock signal;

a second subtractor, for forming a second error signal representing adifference between the filtered first error signal and the secondfrequency comparison signal;

a second digital filter, for receiving the second error signal andforming a filtered second error signal; and

a numerically controlled oscillator, for receiving the filtered seconderror signal and generating the output clock signal.

According to a second aspect of the invention, there is provided anaudio processing integrated circuit, comprising:

-   -   at least one digital audio interface, for receiving digital        audio data with an accompanying audio data clock;    -   a digital-analog converter, for reconstructing analog audio data        based on received digital audio data; and    -   a clock generator according to the first aspect, wherein the        audio data clock is provided to the clock generator as the first        input clock signal, and the output clock signal of the clock        generator is used as the clock for the digital-analog converter.

According to a third aspect of the invention, there is provided a devicecontaining an audio processing integrated circuit according to thesecond aspect, and further comprising:

a communications processor for receiving cellular calls, the processorbeing coupled to the at least one digital audio interface via a digitalaudio bus; and/or

an applications processor for retrieving audio data from local memory,the processor being coupled to the at least one digital audio interfacevia a digital audio bus; and/or

a wireless modem for transmitting and/or receiving audio data from aperipheral device, the modem being coupled to the at least one digitalaudio interface via a digital audio bus.

According to a fourth aspect of the invention, there is provided anintegrated circuit, comprising:

-   -   a clock generator according to the first aspect;    -   a first oscillator, for generating a first oscillator clock        signal, and being connected to the first clock signal input of        the clock generator; and    -   a second oscillator, for generating a second oscillator clock        signal, and being connected to the second clock signal input of        the clock generator,    -   wherein the first and second oscillators are configured such        that the first oscillator clock signal has lower jitter than the        second oscillator clock signal, and the second clock signal has        higher frequency accuracy than the first oscillator clock        signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how itmay be put into effect, reference will now be made, by way of example,to the accompanying drawings, in which:—

FIG. 1 is a schematic diagram illustrating the general form of afrequency generator in accordance with the invention;

FIG. 2 shows in more detail a frequency generator as shown in FIG. 1;

FIG. 3 is a frequency plot, illustrating the properties of the frequencygenerator of FIG. 2;

FIG. 4 shows a first alternative frequency generator in accordance withthe invention;

FIG. 5 shows a second alternative frequency generator in accordance withthe invention;

FIG. 6 shows a third alternative frequency generator in accordance withthe invention;

FIGS. 7( a), 7(b), 7(c), 7(d), 7(e), and 7(f) show in more detailpossible forms of the numerical controlled oscillator in the frequencygenerator in accordance with the invention;

FIG. 8 shows a first system including a frequency generator inaccordance with the invention;

FIG. 9 shows a first possible form of the delta-sigma modulator in thesystem of FIG. 8;

FIG. 10 shows a second possible form of the delta-sigma modulator in thesystem of FIG. 8;

FIG. 11 shows a third possible form of the delta-sigma modulator in thesystem of FIG. 8;

FIG. 12 shows a first known audio processing channel;

FIG. 13 shows a second known audio processing channel;

FIG. 14 shows an audio processing channel including a frequencygenerator in accordance with the invention;

FIG. 15 shows a second system including a frequency generator inaccordance with the invention;

FIG. 16 shows a third system including a frequency generator inaccordance with the invention;

FIG. 17 shows a fourth system including a frequency generator inaccordance with the invention; and

FIG. 18 shows a fifth system including a frequency generator inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a clock generator (or equivalently a frequency generator orfrequency synthesiser) 10, which operates using the principles of afrequency locked loop. The clock generator 10 can for example beprovided in the form of an integrated circuit, or it can be provided asa functional block as part of a larger integrated circuit. The clockgenerator 10 receives a first input clock signal DCK, having a frequencyf_(DCK), and a second input clock signal RCK, having a frequencyf_(RCK), and generates an output clock signal CKout, having a frequencyf_(CKout). The clock generator 10 also receives, at a frequency controlinput FC, an input value or frequency control word P, which representsthe desired value of the ratio of the frequencies of the output clocksignal CKout and the first input clock signal DCK. Thus, where a userwishes to generate an output clock signal CKout having a frequencyf_(CKout), and has a clock signal DCK at a frequency f_(DCK) available,the value of P is set to be equal to f_(CKout)/f_(DCK).

The output clock signal CKout is applied to the clock input (CK) of afirst counter 12, acting as a first frequency detector or firstfrequency comparator. The first input clock signal DCK is applied to thereset (RST) input of the first counter 12. Thus, the first counter 12counts the number of pulses of the output clock signal CKout that aregenerated in each pulse of the first input clock signal DCK, and thecount value CNT1 output from the first counter 12 is a first frequencycomparison signal, representing the ratio of the frequencies of theoutput clock signal CKout and the first input clock signal DCK, i.e.CNT1=f_(CKout)/f_(DCK).

The input value P input at the frequency control input FC is applied toa first input of a first subtractor 14, and the count value CNT1 outputfrom the first counter 12, i.e. the first frequency comparison signal,is applied to a second input of the subtractor 14. The resulting firsterror signal Merr1 is applied to the input (IN) of a first digitalfilter 16, which is clocked by the first input clock signal DCK, andgenerates on its output (OUT) a first filtered error signal Nerr1. Firstdigital filter 16 may be an integrator or some other type of low passfilter with higher gain at low frequencies and lower gain at higherfrequencies.

The output clock signal CKout is also applied to the clock input (CK) ofa second counter 18, acting as a second frequency detector or secondfrequency comparator. The second input clock signal RCK is applied tothe reset input (RST) of the second counter 18. Thus, the second counter18 counts the number of pulses of the output clock signal CKout that aregenerated in each pulse of the second input clock signal RCK, and thecount value CNT2 output from the second counter 18 is a second frequencycomparison signal, representing the ratio of the frequencies of theoutput clock signal CKout and the second input clock signal RCK i.e.CNT2=f_(CKout)/f_(RCK).

The first filtered error signal Nerr1 is applied to a first input of asecond subtractor 20, and the count value CNT2 output from the secondcounter 18, i.e. the second frequency comparison signal, is applied to asecond input of the second subtractor 20. The resulting second errorsignal Merr2 is applied to the input (IN) of a second digital filter 22,which is clocked by the second input clock signal RCK, and generates onits output (OUT) a second filtered error signal Nerr2. The seconddigital filter 22 may be an integrator or some other type of low passfilter with higher gain at low frequencies and lower gain at higherfrequencies

The second filtered error signal Nerr2 is passed to a numericallycontrolled oscillator (NCO) 24, which generates the output clock signalCKout in dependence on its input signal Nerr2.

In some embodiments the digital filters 16 and 22 are integrators, orother type of low pass filters, with higher gain at low frequencies thanat higher frequencies. Thus for bounded output values, the average inputvalue of each integrator 16, 22 must be close to zero. The operation ofthe frequency generator 10 can be understood by considering that, in asteady state, the feedback loops must operate such that the outputsignal Merr1 of the first subtractor 14 tends towards zero, that is,such that:

Merr1=P−CNT1=0, i.e.

P=f_(CKout)/f_(DCK), or

f_(CKout)=P·f_(DCK)

In this embodiment, the value of P is constant, or at least changesrelatively slowly over time compared to any of the generated frequenciesor loop bandwidths, so that the frequency of the output clock f_(CKout)tracks a slowly varying required value.

Similarly, it may be shown that:

Merr2=Nerr1−CNT2=0, i.e.

Nerr1=f_(CKout)/f_(RCK),

i.e. the output of integrator 16 settles out to an average value equalto f_(CKout)/f_(RCK).

In steady state, the output of the second filter 22 will settle out tothe value required by the NCO 24 to provide the desired frequencyP·f_(DCK) at CKout.

FIG. 2 shows in more detail the form of the frequency generator 10, inone specific embodiment.

As shown in FIG. 2, the first counter 12 includes a delay element 30,which is clocked by the output clock CKout, with the result that anadder 32 adds the received value +1 to a current count value for eachpulse of the output clock CKout. A multi-bit storage register 34 (drawnas a single-bit D-type flip-flop for simplicity) receives the currentcount value in each time period, and, on each rising, edge of the firstinput clock DCK, the accumulated count is latched and output to thefirst subtractor 14. At the same time, the rising, edge of the firstinput clock DCK controls a multiplexer 36, so that the value 0 is passedto the adder 32. This resets the counter value of the first counter 12to zero until the next CKout pulse arrives

There are many possible implementations of this counter block 12: forexample the multiplexer could be omitted so that the output of the delayblock continually increments, and then at each rising edge of the firstinput clock DCK the previously delivered output value is subtracted. Ofcourse, this and other blocks could be designed to be triggered onfalling instead of rising edges.

Thus, the accumulated count value CNT1 output to the first subtractor14, i.e. the first frequency comparison signal, is the number of pulsesof the output clock CKout for each period of the first input clock DCK,and is made available to the subtractor 14 for as long as is necessary.

Similarly, the second counter 18 may include a delay element 40, whichis clocked by the output clock CKout, with the result that an adder 42adds the received value +1 to a current count value for each pulse ofthe output clock CKout. A multi-bit storage register 44 receives thecurrent count value in each time period, and, on each rising edge of thesecond input clock RCK, the accumulated count is latched, i.e. output tothe second subtractor 20. At the same time, the rising edge of thesecond input clock RCK controls a multiplexer 46, so that the value 0 ispassed to the adder 42. This resets the counter value of the secondcounter 18. Again alternative implementations of suitable counters arepossible.

Thus, the accumulated count value CNT2 output to the second subtractor20, i.e. the second frequency comparison signal, is the number of pulsesof the output clock CKout for each period of the second input clock RCK,and is made available to the second subtractor 20 for as long as isnecessary.

In the embodiment shown in FIG. 2, the first filter 16 takes the form ofan integrator, comprising an adder 50, a delay element 52, which isclocked by the first input clock DCK, and a multiplier 53, with theresult that, during each period of the first input clock DCK, the valueMerr1 received from the first subtractor 14 is added to the previousrunning total output from delay element 52 to form a new running total,which is then scaled by a factor γ₁, to form the new filter output valueNerr1. The multiplier may just be a bit-shifter, to implement a scalingfactor γ₁ of the form 2^(N), or some other simple shift-and-addstructure to implement a scaling factor γ₁ of the form 2^(N1)+2^(N2) orsimilar, rather than a full multiplier, as is known.

Similarly the second filter 22 takes the form of an integrator ofsimilar structure, comprising an adder 60, a delay element 62, which isclocked by the second input clock RCK, and a multiplier 63, with theresult that, during each period of the second input clock RCK, the valueMerr2 received from the second subtractor 20 is added to the previousrunning total output from delay element 62 to form a new running total,which is then scaled by a factor γ₂, to form the new filter output valueNerr2.

As shown in FIG. 2, the numerically controlled oscillator (NCO) 24 mayinclude a digital-analog converter (DAC) 70 and a voltage controlledoscillator (VCO) 72, and so it generates the output signal CKout at afrequency that corresponds to the numerical value output by the secondintegrator 22.

In this illustrated embodiment, the first integrator 16 is designed tohave a much lower gain (that is to say, a longer time constant) than thesecond integrator 22.

FIG. 3 is a frequency plot, illustrating the jitter transfer functionsbetween various points in the circuit of FIG. 2. Specifically, the plotline 80 represents the jitter transfer function from the DCK input tothe NCO output CKout, i.e. the factor by which any jitter at aparticular frequency present on the DCK input is scaled by when itreaches the output CKout. Similarly, the plot line 82 represents thejitter transfer function from the RCK input to the NCO output CKout, andthe plot line 84 represents the jitter transfer function from the inputof the NCO 24 to its output CKout.

The break points are defined by the unity gain frequencies of the firstand second integrators 16, 22. More specifically, the break point f₈₀ ofthe transfer function from the DCK input to the NCO output CKout, i.e.plot line 80, is given by f₈₀=f(DCK)*γ₁/2π, where γ₁ is the gain of thefirst integrator 16, and the break point f₈₂ of the transfer functionfrom the RCK input to the NCO output CKout, i.e. plot line 82, is givenby f₈₂=f(RCK)*γ₂/2π, where γ₂ is the gain of the second integrator 22.

In this example, these gain values are set such that the break point f₈₀is at approximately 2 Hz (12 rad/s), while the break point f₈₂ is atapproximately 20 kHz (120 krad/s).

As the integrators 16, 22 are both first-order integrators, the slopesof the jitter transfer functions 80, 82, 84 are generally 20 dB/decade,except that the NCO transfer function 84 has a slope of 40 dB/decadebelow the lower break point f₈₀, and the RCK transfer function 80 has aslope of minus 40 dB/decade above the upper break point f₈₂.

Thus at intermediate jitter frequencies, between the two break pointsf₈₀ and f₈₂, the long time constant of the first integrator 16 will tendto keep its output Nerr1 constant, and so very little jitter from thefirst clock signal DCK will propagate to the output, whereas the secondintegrator 22 will have high gain, and thus the feedback loop comprisingit and frequency comparator 18 will control the NCO 24 so that itsoutput CKout follows the second clock signal RCK, despite the inherentjitter of the NCO 24.

Thus, at these intermediate frequencies, the jitter on the output clockCKout will tend to follow the jitter on the second clock signal RCK, andjitter on the first clock signal DCK will be attenuated by 6 dB at 4 Hzand much more at higher frequencies.

As a result, the circuit shown in FIG. 2 is particularly useful insituations where there are available two clock signals, with one havinga frequency that it is desired to use as the basis for generating anoutput clock signal CKout, but which is of relatively low quality (i.e.it has relatively high jitter), and the other being of relatively highquality (low jitter) but an inappropriate frequency. In that case, therelatively high jitter clock signal can be used as the first input clocksignal DCK to the circuit 10, and the relatively low jitter clock signalcan be used as the second input clock signal RCK.

This means that the output clock signal CKout can be generated from thefirst input clock signal DCK at a desired exact frequency ratio bysuitable choice of a value for P, but that (within the frequency band ofinterest) the jitter on the output clock signal CKout is only dependenton the low level of jitter on the high quality second input clock signalRCK, provided that the gains of the integrators 16, 22 are setappropriately.

As mentioned above, in this illustrated embodiment, the first integrator16 is designed to have a much longer time constant than the secondintegrator 22. However, the first integrator 16 could have a shortertime constant than the second integrator 22.

As mentioned above, the integrators 16, 22 are both first-orderintegrators in this embodiment. However, it will be appreciated thathigher order integrators could be used, or indeed other filtersdepending on the anticipated jitter spectrum of the input clocks or thejitter spectrum desired at the output. For instance, it may be desiredto notch out known interference frequencies, in which case filter 16might be a notch filter and filter 22 a bandpass filter, with parametersdesigned to maintain stability of the two feedback loops.

As described above, the integrator 16 settles out to an average valueequal to f_(CKout)/f_(RCK). On start-up, typically all circuitry will bereset to zero, so this low-bandwidth integrator may take some time toramp up to the required value.

FIG. 4 shows an alternative frequency generator 90, in which many of thecomponent blocks are the same as the component blocks of the frequencygenerator 10. These component blocks are indicated by the same referencenumerals in FIG. 4 as in FIGS. 1 and 2, and are not described further.

In the frequency generator 90 shown in FIG. 4, the count (CNT2) outputof the second counter 18 is applied to a first input of a thirdsubtractor 92, and an input value Q equal to the expected or nominalvalue of f_(CKout)/f_(RCK) is applied to a second input of the thirdsubtractor 92 to be subtracted from CNT2. A frequency comparison signalCNT2X is obtained as the result of this subtraction, and this is appliedto the second subtractor 20 to be subtracted from the filtered errorsignal Nerr1.

In operation, the output value CNT2 of second counter 18 must still onaverage be equal to f_(CKout)/f_(RCK), but subtracting Q from CNT2 willgive a value close to zero, if RCK is near the expected frequency. Thenew error signal CNT2X forwarded to the second subtractor will now besmall, so the first integrator 16 need only ramp up to settle out at amuch smaller value, i.e. just the error (f_(CKout)/f_(RCK)−Q) in theestimated Q rather than the full amount of Q. Thus the time taken tosettle to an acceptable accuracy may be greatly reduced.

As an equivalent alternative, the second counter 18 could be preset to acount value of −Q, so that it then ramps up to a value close to zeroduring each RCLK cycle, rather than using an explicit subtractor 92. Asa further equivalent alternative, the output of integrator 16 could bepreset to Q on start-up.

FIG. 5 shows a further alternative frequency generator 110, in whichmany of the component blocks are the same as the component blocks of thefrequency generator 10. These component blocks are indicated by the samereference numerals in FIG. 5 as in FIGS. 1 and 2, and are not describedfurther.

In the frequency generator 110 shown in FIG. 5, the output clock CKoutis not passed directly to the first counter 12 and the second counter18, but is instead passed through a common frequency divider 116 andthen separately through a first frequency divider 112 and a secondfrequency divider 114 respectively. The common frequency divider 116receives a common frequency division ratio N₀ and generates a commonfrequency divided clock signal CKout0 at a frequency f_(CKout)/N₀. Thefirst frequency divider 112 receives a first frequency division ratio N₁and generates a first frequency divided clock signal CKout1 at afrequency f_(CKout)/N₀N₁, which it supplies to the clock (CK) input ofthe first counter 12. Similarly, the second frequency divider 114receives a second frequency division ratio N₂ and generates a secondfrequency divided clock signal CKout2 at a frequency f_(CKout)/N₀N₂,which it supplies to the clock (CK) input of the second counter 18.

The common, first, and second frequency dividers 116, 112, 114 may befixed dividers, and they have the effect that the first and secondcounters 12, 18 can run more slowly than if they are supplied with theoutput clock CKout directly, which has the advantages that powerconsumption is reduced, and that the counters 12, 18 are only requiredto be able to count to smaller values, without seriously affecting theaccuracy of the frequency f_(CKout) or the jitter of the output clockCKout. If required, the common frequency divided clock signal Ckout0,first frequency divided clock signal CKout1 and/or the second frequencydivided clock signal CKout2 can be provided as a clock signal to otherblocks of a larger circuit. Depending on requirements, one of more offrequency dividers 112, 114, 116 may be unnecessary and omitted.

Alternatively, one or more of these frequency dividers may beconfigurable during operation, by altering the values of N1 and/or N2,to accommodate different modes of operation with different input clockfrequencies or different system requirements. The frequency dividers maybe implemented as chains of divide-by-2 stages to implement frequencydivision ratios of the form 2^(N), or may be a counter including decodeand appropriate reset logic or other similar techniques as known.

In some applications, one or both of the input clock signals (DCK, RCK)may not always be present. It is desirable that some output clock isstill maintained in such circumstances, to allow at least somecontinuing operation of parts of the host system.

In the embodiments described above, if the first input clock signal DCKgoes away, while the second input clock signal RCK is still present,then the filtered error signal Nerr1 becomes constant, and the NCO 24 iscontrolled only by the loop including the second counter 18. This loopwill maintain the output frequency at a frequency based on the frozenvalue of Nerr1 and the frequency of RCK.

If, alternatively, in the embodiments described above, the second inputclock signal RCK goes away, the second digital filter 22 will becomeunclocked, and so its output Nerr2 will remain at its last value. Thiswill cause the NCO output frequency to become constant, at a frequencycorresponding to this value.

FIG. 6 shows a further alternative frequency generator 130, in whichmany of the component blocks are the same as the component blocks of thefrequency generator 10. These component blocks are indicated by the samereference numerals in FIG. 6 as in FIGS. 1 and 2, and are not describedfurther.

In the frequency generator 130 shown in FIG. 6, a first clock detector132 is connected to the input receiving the first clock signal DCK, andis used to control a first limiter block 134. Similarly, a second clockdetector 136 is connected to the input receiving the second clock signalRCK, and is used to control a second limiter block 138.

The clock detectors 132, 136 and limiter blocks 134, 138 are provided todeal with issues that can arise if the input clocks become temporarilyunavailable. For example, if the first clock signal DCK goes away, itmay have a slightly different frequency when it reappears (perhapsbecause a frequency of an upstream clock generator has drifted with avariation in temperature, or perhaps even because it is being derivedfrom a different clock source). If the second clock RCK goes away, theNCO 24 runs open-loop for a while and its frequency may have drifted bythe time that the RCK signal reappears. The step changes in effectiveinput clock frequencies due to the sudden reimposition of the clocks maygive large step transients in the frequency comparison signals which maystimulate the feedback loops to produce a noticeable output frequencytransient overshoot.

The frequency generator 130 shown in FIG. 6 helps to mitigate theseproblems in that the clock detector 132, 136 detects when the respectiveclock signal DCK, RCK is not present, and then detects when itreappears. The clock detector 132, 136 then limits the input to therespective digital filter 16, 22 during a recovery period after theclock signal reappears, in order to help reduce any large-signaltransient overshoots within the loops and thus reduce any transientovershoots in the output frequency.

The numerically controlled oscillator (NCO) 24 can take many possibleforms. FIG. 2 shows an NCO 24 comprising a voltage-output DAC 70 drivinga voltage-controlled oscillator 72. As one alternative, FIG. 7( a) showsan NCO in the form of a simple RC oscillator 440 with digitallyprogrammable resistor elements 442 and capacitor elements 444, whileFIG. 7( b) shows an NCO in the form of a transconductance/capacitor(Gm-C) ring oscillator 446, again with digitally programmable resistorelements 448 a, 448 b, 448 c and capacitor elements 450 a, 450 b, 450 c.

FIG. 7( c) shows in more detail one possible form of the programmablecapacitor elements 444 or 450, having multiple capacitors 452 a, 452 b,452 c connectable in parallel combinations by means of switches 454 a,454 b, 454 c, these switches comprising MOS transistors or othersuitable active devices. In other embodiments, some or all of thecapacitors making up a programmable capacitor element can be connectablein series to other capacitors.

FIG. 7( d) shows in more detail the form of the programmable resistorelement 442, having multiple resistors 456 a, 456 b, 456 c, 456 d,connectable in series or in parallel combinations by means of switches,these switches comprising MOS transistors or other suitable activedevices.

Thus, In FIGS. 7( c) and 7(d), respective bits of a binary signalrepresenting the numerical input or derived therefrom can control whichof the capacitors and resistors are connected, and can thus control thefrequency of the oscillator.

FIG. 7( e) shows an alternative NCO, in which the oscillator element isa ring oscillator 458 where the supply current is digitallycontrollable. (Similarly, the supply voltage or other bias voltage of aring oscillator might be digitally controllable.) Thus, in FIG. 7( e),the ring oscillator 458 comprises three inverting stages 460 a, 460 b,460 c (possibly simple CMOS inverter stages), although of course anyconvenient number of such inverters can be provided. Three PMOStransistors 462 a, 462 b, 462 c are provided in an array, and respectivebits of a binary signal representing the numerical input can controlwhether the gates of the PMOS transistors are connected to a suitablebias voltage V_(B) or to a supply voltage V_(DD). Gates that areconnected to the bias voltage V_(B) contribute to the total currentI_(ctrl) that is available to drive the inverter chain and thus controlthe supply voltage V_(ctrl) and output frequency of the ring oscillator458.

The output of the digital filter will however typically be 20 to 30 bitswide, whereas it is impracticable to provide enough passive elements toprovide such a high resolution directly. In order to mitigate thisproblem, word length reduction techniques may be used.

FIG. 7( f) shows a possible form of numerically controlled oscillator(NCO) 140, which can be used in place of the numerically controlledoscillator (NCO) 24 described above. The numerically controlledoscillator (NCO) 140 comprises a digital-analog converter (DAC) 70 and avoltage controlled oscillator (VCO) 72, which have the same functions asthe blocks indicated by the same reference numerals in FIG. 2, and thesewill not be described further. In the numerically controlled oscillator(NCO) 140, the value output by the second digital filter 22 is passedfirst to a word-length reduction (WLR) circuit 142, such as anoise-shaper or a delta-sigma modulator. This helps to ensure that anynoise that results from quantising the value is pushed out to higherfrequencies, where it will not produce any noticeable effects in theoutput jitter at low and medium frequencies. It is the output of theword-length reduction (WLR) circuit 142 that is then passed to thedigital-analog converter (DAC) 70.

The output of the digital-analog converter (DAC) 70 may be passed to ananalog low-pass filter 144 to further help attenuate quantisation noiseintroduced by the DAC 70, and to help decouple any other high frequencynoise against the local ground of the VCO. It is the low-pass filteredsignal that is then passed to the voltage controlled oscillator (VCO)72, to generate the output signal CKout at a frequency that correspondsto the numerical value output by the integrator 22.

This allows a relatively low resolution (perhaps 6-bits) DAC to have amuch finer effective d.c. resolution (say 16 bits) in terms of theaverage value applied to the VCO and the respective output frequencyaccuracy.

Similarly a word length reduction circuit may be used to control theprogrammable resistor and/or capacitor elements in the structures shownin FIG. 7( a) or 7(b), or the PMOS transistors in the structure shown inFIG. 7( e). In the case of FIG. 7( e), the capacitor C_(filt) will actas a low-pass filter element.

Thus far, the value of P applied to the frequency control input FC hasbeen assumed to be constant over time, or at least to be changing slowlyover time compared to any of the generated frequencies or loopbandwidths, so that the frequency of the output clock tracks a slowlyvarying required value corresponding to P.

Alternatively, the variation can be fast enough, that is, in excess ofthe FLL loop bandwidth, that the effect of the first and second low passfilters 16, 22 is to cause the average frequency of the output clockf_(CKout) to depend on an average value of P, and for any modulation ofthis frequency to be insignificant. For instance P may alternate rapidlybetween two or more adjacent values such that their average representsthe desired frequency, or the input value P applied to the first inputof the first subtractor 14 may be any sequence or stream of valueshaving desired properties in terms of its average value and quantisationnoise spectrum, and can, for example, be the stream of multi-bit orone-bit words output from a multi-bit or one-bit sigma-delta modulator.

FIG. 8 shows a further alternative frequency generator 120, in whichinputs having numerical values “a” and “b” are input to a modulator 122,for example a delta-sigma modulator, to generate a stream of outputvalues having the average value a/b. An exact fractional average outputvalue can therefore be obtained by supplying integer values a and b. Thestream of output values is supplied to a frequency generator sub-block124 to serve as a frequency control word applied to the input FC. Thefrequency generator sub-block 124 can take the form of any of thefrequency generators described herein, namely the frequency generators10, 90, 110, 130, and the stream of output values is supplied as theinput desired value P for the frequency ratio. Thus, if it is desired togenerate an output clock signal at a frequency that is an exactfractional multiple of the frequency of an available clock, themodulator 122 can be used to generate the desired value for the multipleby choosing suitable values of a and b, and this can be supplied as thedesired multiplication factor, P, in the form of the stream of modulatoroutput values to the frequency generator sub-block 124.

FIG. 9 illustrates a first order delta sigma modulator 330 for use asthe modulator 122 in an embodiment of the present invention. Delta sigmamodulator 330 comprises a first input terminal 331 for receiving a firstinput value a. An addition element 332, which may be an adder, receivesthe first input value a from the first input terminal 331. A memoryelement 334 is coupled to the output of the addition element 332 andmaintains an accumulated count. The output of the memory element 334 iscoupled to a modulus element 336 and a comparison element 338, which maybe a comparator. Delta sigma modulator 330 also comprises a second inputterminal 335 for receiving a second input value b that is provided tothe modulus element 336 and the comparison element 338. Comparisonelement 338 provides a first output, m, on one output line 339 andmodulus unit 336 provides a second output, ε, on a second output line340, which is also fed back to the addition element 332.

Thus, delta sigma modulator 330 has two input terminals 331, 335 forreceiving first and second input values a and b. Input a represents thenumerator of the fractional input and input b represents the denominatorof the fractional input. The first and second input values a and b aretherefore chosen such that a/b=P.

Preferably, although not necessarily, input values a and b are selectedso that they are coprime, i.e. they have no common factors other thanone, in order to eliminate tones in the output of the delta sigmamodulator 330. If input values a and b are not coprime, tones that maybe present in the output may compromise performance, particularly inaudio systems. However other non-audio systems may be capable oftolerating such a compromise.

The value of b can be represented by a binary word, and in particularthe binary word may have more than one non-zero bit, and thus b need notbe of the form 2^(N).

In delta sigma modulator 330, the addition element 332 adds the firstinput value a to the accumulated count until the comparison unit 338determines that the accumulated count maintained in the memory element334 is greater than or equal to a threshold value, b. When theaccumulated count is greater than or equal to the threshold value, b, anoutput pulse is produced at output 339 and the counter is reset by themodulus element 336. That is, the modulus element performs a modulusoperation on its inputs, subtracting the value b from the count valuereceived from the memory element 334, repeatedly if necessary, until theresult is less than b. Thus, in delta sigma modulator 330, the thresholdvalue is set by the second input value b, which sets the modular pointof the delta sigma modulator.

The modular point, or rollover point, of the delta sigma modulator isthe threshold value, b, which when exceeded by the accumulated count,causes a pulse to be outputted at output 339, and the count to be reset.

The ability to set the modular point to the second input value b,enables the delta sigma modulator to synthesise according to base b. Theaverage value of the pulse density of output 339 is therefore exactlyequal to a/b, because the synthesis is quantised to b which is thedenominator of the input a/b. The ability to use a value of b that isnot of the form 2^(N) allows a much wider range of frequencies to besynthesised exactly, rather than with a ratio rounded to a certainnumber of binary places.

In the situation where the accumulated count exceeds the thresholdvalue, the excess that the accumulated count is greater than thethreshold value is carried over (rolled over) as a remainder at thestart of the next addition cycle.

Output terminal 340 outputs the error value, c, of the delta sigmamodulator 330. The error value is reduced in higher order delta sigmamodulators because it is cascaded to subsequent delta sigma modulationstages. In first order delta sigma modulators, such as the delta sigmamodulator 330 shown in FIG. 9, the error value is discarded.

The present invention is not limited to first order delta sigmamodulators and can also be used in higher order modulators. A higherorder modulator contains at least two delta sigma modulation stages anda recombination stage at the outputs of the stages to combine theoutputs to produce a pulse density modulated output.

Traditionally, modulus operations such as the one performed by modulusunit 336 are computationally expensive. FIG. 10 shows an alternativedelta sigma modulator 341 for use in an embodiment of the presentinvention, in which the modulus unit is simplified to a multiplexer 342and a subtractor 344.

Delta sigma modulator 341 operates in a similar manner to that describedin relation to delta sigma modulator 330 of FIG. 9. The first inputvalue a from input terminal 331 is added by the addition element 332 toan accumulated count maintained in memory element 334. The accumulatedcount is inputted to the multiplexer 342, along with the accumulatedcount minus an input value b, inputted from input terminal 335, fromsubtractor 344. The multiplexer 342 selects an output from one of theseinputs depending on whether the comparison element 338 determines thatthe accumulated count is greater than or equal to input value b. Ifcomparison element 338 determines that the accumulated count is greaterthan or equal to input value b, comparison element 338 outputs a pulseat output 339, which is also passed to the multiplexer 342 indicatingthat multiplexer 342 should select the output with the accumulated countminus input value b for feed back to the addition element 332. Thissimplified circuit is an acceptable replacement for the circuit shown inFIG. 9, if it is known that the relationship between the values of a andb will be such that repeated subtraction of the value b will not berequired. Being a simplified circuit, this will have the advantage of asmaller chip area and hence lower cost, and also have lower powerconsumption.

FIG. 11 shows an example of a third order delta sigma modulator 400 inaccordance with an embodiment of the present invention. Delta sigmamodulator 400 comprises a first input terminal 401 for receiving a firstinput value a. Addition element 402 receives the first input value afrom the first input terminal 401. A first delta sigma modulation stage404, which may take the form of the delta sigma modulator 330 shown inFIG. 9 or the delta sigma modulator 341 shown in FIG. 10, is coupled tothe output of the addition element 402. A first output 405 of the firstdelta sigma modulation stage 404, which is a pulse density modulatedsignal the average of which is m ₁, is coupled to a recombination stage420. The second output 406 of the first delta sigma modulation stage 404is the error £₁ of the first delta sigma modulation stage 404, which iscoupled to the input of a second delta sigma modulation stage 407, whichmay take the form of the delta sigma modulator 330 shown in FIG. 9 orthe delta sigma modulator 341 shown in FIG. 10. A first output 408 ofthe second delta sigma modulation stage 407, which is a pulse densitymodulated signal the average of which is m ₂, is coupled to therecombination stage 420. The second output 409 of the second delta sigmamodulation stage 407 is the error ε₂ of the second delta sigmamodulation stage 407, which is coupled to the input of a third deltasigma modulation stage 410, which may take the form of the delta sigmamodulator 330 shown in FIG. 9 or the delta sigma modulator 341 shown inFIG. 10. An output 411 of the third delta sigma modulation stage 410,which is a pulse density modulated signal the average of which is m ₃,is coupled to the recombination stage 420.

Delta sigma modulator 400 also comprises a second input terminal 434 forreceiving a second input value b. Though not shown in FIG. 11, thesecond input value b is provided to delta sigma modulation stages 404,407, 410. Second input value b determines the modular point of the deltasigma modulation stages, as discussed previously.

Recombination stage 420 combines the outputs m ₁, m ₂, and m ₃ from thethree modulation stages 404, 407, 410 respectively to produce an output432, in a manner which is in itself known to the person skilled in theart, and will therefore not be described further herein.

A dither unit 430, which receives a dither control signal, appliesdither to the addition element 402. Dither is an intentionally appliedform of noise that is applied in order to randomize quantisation errorand prevent or at least greatly reduce tones forming in the output ofdelta sigma modulator 400. Dither is introduced to eliminate tones inthe power/frequency response of the delta sigma modulator in a mannerwhich is in itself known to the person skilled in the art, and willtherefore not be described further herein.

There has thus been described a clock generator, with variousadvantageous properties.

Such clock generators as described above are useful in a consumer/hostdevice which includes audio reproduction or other audio signalprocessing, such as, but not limited to, a smartphone, games console,tablet, laptop computer, desktop computer, hi-fi system, or the like. Insuch applications, it is necessary for good audio reproduction to use aclock that has low jitter, and is at exactly the same frequency as theincoming data rate (or at a multiple thereof), to avoid dropping oradding data samples.

FIG. 12 shows a known audio processing channel 150, in this casepossibly an audio reproduction channel accepting input digital data andoutputting an analog signal suitable for directly or indirectly drivinga speaker or headphone. The incoming data is transmitted in accordancewith a clock or clocks (for example a frame clock and a bit clock in thecase of a serial data format). One of these clocks is used to captureincoming data in an Audio Interface (AIF) 151. This data may thenundergo some digital signal processing (for example interpolation ordecimation) in a digital signal processing block (DSP) 152 to produceprocessed data Data′, before being output through a delta-sigma DAC 154,the DAC and DSP being clocked according to some processing clock orclocks.

The processing clock may be just a (buffered) version of one of theinterface clocks (as shown by the dashed line 155), or may pass througha frequency generator 156, for example to multiply an incoming frameclock LRCLK at say 48 kHz to a higher-speed clock (say 3072 kHz) CKsysthat can be used to clock the output DAC 154.

However, the incoming clock may be subject to some jitter due todistortion or noise added in transmission, or a clock generatoravailable on a digital processor chip may be of low quality. Thisjitter, when apparent on the sampling clock of the DAC, may cause pooraudio quality in terms of noise, distortion or spurious intermodulationproducts.

FIG. 13 shows an alternative audio processing channel 150, in which theDAC clock CKsys is derived from a local clock generator 157. However,particularly in systems with multiple channels from multiple sources,the local clock CKsys may not be synchronised to, or may not be the sameactual frequency as, the incoming clock, even if it is the same nominalfrequency. To cope with possible asynchronism between the incoming dataand the local clock, an asynchronous sample-rate converter block 158needs to be included in the signal chain, for example interpolating orup-sampling the input data up to a very high sample rate, smoothing itthough a filter, and then decimating or down-sampling it to a desiredsample rate. This takes up substantial digital hardware and powerconsumption, and may degrade the audio quality, particularly say if theinput data is companded or otherwise non-linearly coded.

When the audio reproduction device 150 is in use in a consumer devicesuch as a smartphone, PDA or digital camera or the like, that consumerdevice might include a good quality, low jitter, clock for its otherfunctions, for example from a crystal generating a clock signal at 19.2MHz or 12 MHz.

FIG. 14 shows a configuration of an audio processing channel 150 thatcan advantageously be used in such circumstances, including a frequencygenerator 159, which may take the form of any of the frequencygenerators described herein, namely the frequency generators 10, 90,110, 130, or similar.

In this case, the input data is accompanied by a low quality clock CKIN,for example an audio word clock LRCLK, whose nominal frequency is afraction 1/P of the frequency of the output clock CKout. The clock CKINcan then be supplied to the first clock (DCK) input of the frequencygenerator 159, while the high quality clock from the crystal (XTAL) canbe supplied to the second clock (RCK) input of the frequency generator159. By suitable choice of the input values, as described above, thefrequency generator 159 can be made to generate a clock signal CKoutthat is synchronised to the long term frequency of the incoming LRCLK,but preserves the low audio band jitter of the clock signal from thecrystal (XTAL). This clock signal can then be supplied to each of theDSP 152 and DAC 154 decoding each of the channels of audio data. It isnot necessary to provide an asynchronous sample rate converter (ASRC),at least on this one channel, or any other channels synchronised to it,e.g. the other one of a stereo pair, or other channels, with commonLRCLK. As a result, this configuration compares favourably in terms ofpower, area etc with alternatives that require a low-jitter clock to beprovided for the DAC and/or require asynchronous sample rate conversionto allow for different clock bases to maintain accurate frequency and toavoid dropping data samples.

In practice, the clock actually delivered to the DAC and/or DSP may be adivided-down version of the frequency generator output, but suchpossible dividers are omitted from the drawing for simplicity.

FIG. 15 shows the form of an alternative and bidirectional audioprocessing channel 160. Thus, while an Audio Interface (AIF) 161receives digital data, which it passes to a digital signal processingblock (DSP) 162 to produce processed data Data′ that is output through aDAC 164, an ADC 163 receives analog data, which it converts to digitalform, before processing in the DSP 162 and output via the AIF 161. TheADC 163, as well as the DSP 162 and DAC 164, are clocked using the clocksignal CKsys output from the frequency generator 159.

FIG. 16 shows the use of a frequency generator as described, in a systemwith multiple audio channels and associated clocks. Specifically, FIG.16 shows an audio hub circuit 202, which may be implemented as anintegrated circuit, as used in a consumer host device 203 such as asmartphone or a portable audio convergence device for example.

Mobile telephony signals from and to an RF front end 204 are coupledthrough first processing circuitry, say a cellular communications(“comms”) processor 206, comprising an audio interface (AIF) 207 tocommunicate these signals in the form of streams of sampled digitalaudio data to and from a first audio interface 208 on the audio hub 202.The comms processor 206 contains clock generation circuitry 210, whichis synchronised to the RF receive/transmit channel, i.e. to the externalphone network. The sampling rate of the audio data streams between thecomms processor and the audio hub is thus synchronised to the externalnetwork.

The consumer/host device 203 may also playback or record audio signalsstored in a local memory or removable media 224, coupled to a secondinterface 226 on the audio hub 202 via second processing circuitry, sayan applications (“apps”) processor 228, with associated clock circuitry230 and comprising an audio interface 232 to communicate these signalsin the form of streams of sampled digital audio data to and from thissecond interface. The sample rate associated with clocking the audiodata into and out of the storage medium 224 need not necessarily besynchronised to any external reference clock: the sample rate needs onlyto be accurate and stable enough for people not to notice an error inpitch. The sampling rate of the audio data streams between the appsprocessor and the audio hub thus need not be synchronised to anyexternal network or other external source.

The host/consumer device 203 may also have other sources of audio data,for example a Bluetooth™ transceiver, an FM-radio, or a Wi-Fitransceiver, or a High-Definition Multimedia Interface (HDMI), an S/PDIFinterface, or a USB interface, represented in this case by the PHY block234, channeled though further processing circuitry 236 comprising anaudio interface 238 to communicate this audio data in the form ofstreams of sampled digital audio data to and from a third audiointerface 240 on the audio hub 202. These further audio channels mayneed to be synchronised to external circuitry or networks in some cases,in other cases this may not be necessary. The sampling rate of the audiodata streams between the PHY block and the audio hub may thus need to besynchronised to an external network or other external source in some usescenarios, but not in others.

The audio hub 202 also includes a DSP mixer block 260, which maycomprise signal routing, mixing, conditioning and other DSP functions.Signals from the audio interfaces 208, 226, 240 or from ADC analoginterfaces 268 a, 270 a, 272 a (possibly after processing by optionaldedicated DSP blocks 262 a, 264 a, 266 a) can be mixed, conditioned orotherwise processed by DSP block 260, and resultant signals output viathe audio interfaces 208, 226, 248 or from DACs 268 b, 270 b, 272 b(possibly via optional dedicated DSP blocks 262 b, 264 b, 266 b.

The mixer 260, the DSP blocks 262, 264, 266, and the DAC/ADC analoginterfaces 268, 270, 272 are clocked by the clock signal CKsys (orpossibly clock signals divided or otherwise derived from CKsys)generated by the clock generator 256, which may be comprised on the sameaudio hub integrated circuit.

The processing of all audio data streams within DSP block 260 preferablymust be synchronous to the common clock CKsys, despite the diverse audiodata sample rate synchronisation requirements arising from the variousconnected digital audio sources. So, preferably, the frequency generatorcircuit 256 may take the form of any of the frequency generatorsdescribed herein, namely the frequency generators 10, 90, 110, 130, orsimilar.

In this illustrated embodiment, the clock signals generated by the commsprocessor 206 and by the apps processor 228 are passed to a multiplexer258 (or equivalent arrangement). When the device is receiving a cellulartelephone call, the multiplexer 258 is controlled so that the clocksignal generated by the clock generator 210 on the comms processor 206is passed to the DCK input of the frequency generator 256. The frequencygenerator output clock CKsys is used for processing data coupled throughthe comms processor to maintain data synchronism with the externalnetwork. There may be also a requirement to simultaneously couple datafrom the local storage 224 via the apps processor 228, for instance torecord an incoming call, maybe as a local voice mail function. Since thesampling rate of this data need not be synchronised to any externalclock reference, this data flow can also be based on the same clockCKsys, rather than using say some clock generated on the apps processor,which would then require an asynchronous sample rate conversion of theaudio data stream in order to process synchronously with CKsys.

A telephony communications type device may, for example, also processaudio data streams when not receiving a network call, e.g., for MP3/4file playback. In this case the cellular comms processor 206 ispreferably turned off to save power, and the main clock for the systemmay be generated on the apps processor 228, or possibly some other chipin the system, say a Power Management IC (PMIC) 280. Thus, in thissituation, the multiplexer 258 is controlled so that the clock signalgenerated or used by the apps processor 228 is passed to the DCK inputof the frequency generator 256.

The communications type device may also be required to process audiodata streams to/from the other processor 236. In the embodiment of FIG.16, to allow for use scenarios where these data streams must besynchronised to some external network or external clock source, whilethe audio hub clock CKsys is based on say the comms processor clock tomaintain audio synchronism with a cellular network, the audio interface240 comprises an asynchronous sample rate converter (ASRC) 242, togenerate or accept an audio data stream synchronised to the CKsys clockfrom the asynchronous clock used by the processor 236.

In alternative embodiments, this ASRC may be separate from the audiointerface, and the hub may comprise routing circuitry to route audiodata streams from other audio interfaces though this ASRC, whileallowing a path directly between the interface 240 and the DSP block260, and possibly with a third input to the multiplexer 258 to pass theclock from the processor 236 to the clock generator 256, for scenarioswhere it is desirable to base CKsys on a clock from processor 236 andpossibly sample rate convert some other channel, or where the audio datapassing though processor 236 does not require to be synchronised to someexternal reference.

Alternative embodiments may have fewer or more channels, each possiblysimilar in nature to one of the three channels described.

Both the comms processor 206 and the apps processor 228 are large,complex digital circuits, so it is likely that any clock emanatingtherefrom will be corrupted by on-chip digital crosstalk and thus havehigh jitter. As mentioned above, the frequency generator 256 has theadvantage of reducing this jitter to allow better audio quality. In anapplication like this, with a plurality of possible main clock sources,there is a further advantage in that, when the DCK input is switched toa different source, there will be no glitches in the output clock, andthe output frequency will also not suddenly change, since it is lockedto RCK, and the input to the NCO 24 in the frequency generator will onlychange slowly, with a time constant of the order of the sub-audiobandwidth of the outer loop. Thus the transition from one system clockto another, for example if recording an incoming voice message to localvoice mail while playing back some MP3 music, will be inaudible to thelistener.

In further system modes, it may be desirable to turn off even the appsprocessor 228, or at least to run it at a very low duty cycle just tokeep the system in some standby mode. In such modes, the DCK input tothe frequency generator may be disabled altogether, or at leastperiodically. As described above, the frequency generator will thencontinue to generate a clock at close to the nominal frequency, based onthe held value of Nerr1 and the RCK frequency. Again, the transitions inand out of this standby mode, or as DCK periodically appears anddisappears, will then occur without sudden change in phase or frequencyof the output clock.

Also note that even if RCK were to temporarily disappear, the NCO wouldstill continue oscillating at a frequency corresponding to the heldvalue of Nerr2, so some clock is still available to maintain somefunctionality in the system. This frequency will still be reasonablyaccurate, within the resolution or error of the held value of Nerr2,albeit possibly drifting eventually due to any change in temperature orsupply voltage.

FIG. 17 shows a device 170, where the data is supplied from a USB source172. The data is supplied through an interface 173 of the device 170 toan audio and/or video codec 174. The USB source 172 provides a burst ofdata every 1 ms or so for example. Typically, the data is buffered intemporary storage circuitry 175 such as a FIFO for example. The buffereddata is passed to a digital-analog converter (DAC) 177, the analogoutput of which can be used, for example, to drive audio out ofspeaker(s) 178 internal to the device or speakers or headphones 179external to the device.

The USB source 172 also provides a clock, typically at a frequency of 12MHz. The clock might be present whenever the USB source 172 is availableand connected to the codec 174, or it might go away between bursts ofdata. When the clock is only available periodically from the USB source172, it is still necessary to have a stable local clock to providecontinuous output data conversion by means of the DAC 177. It is alsonecessary to avoid glitches when the USB clock periodically returns, toavoid audible artefacts.

When the device 170 is a consumer/host device such as a smartphone, or alaptop computer for example, it might include a good quality, lowjitter, clock for its other functions, for example from a crystalgenerating a clock signal at a known frequency. In such circumstances, afrequency generator or clock generator 176 can advantageously be used.The frequency generator 176 can take the form of any of the frequencygenerators described herein, namely the frequency generators 10, 90,110, 130, or similar.

The USB clock can then be supplied to the first clock (DCK) input of thefrequency generator 176, while the clock from the crystal (XTAL) can besupplied to the second clock (RCK) input of the frequency generator 176.A frequency control (FC) word P is also supplied to the frequencygenerator 176.

This has the effect that the clock signal CKout supplied to the codec174 can be derived from the USB clock. However, when the USB clock andthe associated data burst are not present, the output of the integrator16 in the frequency generator 176 will stay constant, and hence theoutput frequency f_(CKout) will remain at the same fixed ratio to thefrequency of the clock from the crystal (RCK). Thus, the clock signalCKout supplied to the codec 174 will remain close to the intendedfrequency f_(CKout).

There may be a small error in the frequency of the output clock CKoutbetween each burst. The first filtered error signal Nerr1 will generallyvary slightly from one DCK cycle to the next due to the finiteresolution of the digital filter (i.e. quantisation noise) plus possiblyany noise injected into the loop by any delta-sigma modulation of P orcycle-by-cycle variation of the synchronism of DCK to CKout. To achievean error of a quarter of a 12 MHz DCK period, over the 1 ms intervalbetween bursts of USB data, requires Nerr1 to have an accuracy of 1 partin 48000. If Nerr1 may be, say, 1/16 of full scale in some conditions,this requires a 20 bit resolution in Nerr1. The size of storagecircuitry 175 may be increased slightly to allow temporary storage of acache of data to accommodate temporary or short term variation in theoutput clock frequency; the long term frequency will be accurate and soerrors in the clock will not accumulate so this storage should neveroverflow if appropriately sized.

In certain applications, it is undesirable to use off-chip components,such as a crystal (XTAL) for example, in order to save space and reducethe bill-of-materials cost for example, so it is desirable to generatethe required accurate and low jitter clock using wholly on-chipcircuitry.

It is difficult to design a wholly on-chip oscillator that is capable ofoutputting a clock signal having a stable frequency together with lowjitter characteristics without having to design complex circuits thatare large in integrated circuit (IC) die area and/or that requirerelatively large amounts of power to operate. Such IC designdifficulties are due to factors such as: IC manufacturing tolerances interms of doping levels and structural dimensions; and variation of ICoperating temperature or supply voltage in subsequent use for example.

In order to overcome these problems, a solution has been devised whereina clock generator is provided with a plurality of clock signalsgenerated wholly on-chip.

FIG. 18 illustrates an example of an integrated circuit 180 comprising aclock generator 182 and first and second on-chip oscillators 186, 184.The clock generator 182 can be any of the clock generators as hereinpreviously described, namely the frequency generators 10, 90, 110, 130,or similar.

The first oscillator 186 may be designed especially so as to have anoutput frequency insensitive to temperature and supply voltage, but withlittle attention paid to jitter. This oscillator may for example be anentirely on-chip RC-based oscillator, using a temperature stable on-chipcapacitance as is commonly available and a zero-temperature-coefficientresistance. The zero-temperature-coefficient resistance may be obtainedby an appropriately designed network comprising a mixture of types ofon-chip resistors of possibly positive and negative temperaturecoefficients. To obtain an initial absolute frequency accuracy, one ormore of the components of the first oscillator 186 may be trimmed, i.e.adjusted, during manufacture or post-manufacture so that the firstoscillator 186 outputs a very accurate desired frequency. Once trimmed,such a time constant type oscillator will have a relatively stable, i.e.relatively accurate, output frequency but may have high jittercharacteristics.

This first oscillator 186 may be one of the types disclosed above andillustrated in FIGS. 7( a) (b) or (e), comprising digitally programmableresistors, capacitors or other elements, the controlling digital bits ofwhich may be stored in on-chip non-volatile memory such as ROM orfusible elements.

The first oscillator 186 is connected to the clock generator 182, sothat the output of the first oscillator 186 is supplied as the firstinput clock signal (DCK) of the clock generator 182. The first inputclock signal DCK is therefore a relatively accurate frequency clocksignal, but with a relatively high level of jitter.

In contrast to the first oscillator, a second oscillator 184 should bedesigned to generate a low jitter clock, but with less attention paid tofrequency accuracy or stability. The second oscillator 184 may be formedby an oscillator comprising a wholly on-chip resonant circuit, forexample an LC-type oscillator comprising an on-chip inductor L and anon-chip capacitance C. Without the impracticability of resorting todesigning complex circuits as discussed above, a relatively low powerresonant oscillator may be designed using an on-chip inductor to providean output clock signal that has low jitter characteristics, but it isdifficult to also simultaneously provide an output frequency that isvery insensitive to temperature and supply voltage. The difficulty isincreased for a wholly on-chip LC oscillator since, in the current art,a resonant circuit comprising an on-chip inductor (L) has a lowerQ-factor than an equivalent resonant circuit comprising an off-chipinductor. The second oscillator will thus generate a clock with lowjitter, but may have a frequency that varies over time.

The second oscillator 184 is connected to the clock generator 182, sothat the output of the second oscillator 184 is supplied as the secondinput clock signal (RCK) of the clock generator 182. The second inputclock signal RCK is therefore a relatively low jitter clock signal, butwith relatively poor frequency accuracy.

As discussed above, the clock generator 182 acts to generate an outputclock signal that has the frequency accuracy of the clock signalprovided by the first oscillator 186, but has the low jittercharacteristic of the clock signal provided by the second oscillator184.

The design of two separate oscillators, each focussed on separateaspects of performance, is much easier, gives a less complex and hencecheaper circuit, and consumes less power, despite the additionalcircuitry of the clock generator, than attempting to satisfy all therequirements with a single oscillator.

As a result, the circuit shown in FIG. 18 is particularly useful insituations where it is undesirable, or not possible for whatever reason,to use off-chip components in the generation of an output clock (CKout)that is stable, i.e. accurate, and that has low jitter. In generating,completely on-chip, two clock signals, wherein one clock signal (DCK)has an appropriate, i.e. desired, frequency and is relatively accurateover time but has relatively high jitter, and the other clock signal(RCK) has an inappropriate frequency that is relatively inaccurate butthat has relatively low jitter characteristics, and using these as theclock inputs to a clock generator as described above, the bestcharacteristics of each of the on-chip generated clock signals (RCK,DCK) is present in the clock generator output signal (CKout).

This means that the output clock signal CKout can be generated from thesecond input clock signal DCK at a desired exact frequency ratio bysuitable choice of a value for P, but that the jitter (within thefrequency band of interest) on the output clock signal CKout is onlydependent on the low level of jitter on the second input clock signalRCK.

It should be noted that the second oscillator could alternatively beformed from some other resonant relatively high-Q element, for example,by means of a MEMS (Micro-Electro-Mechanical-System) type oscillator aspart of the integrated circuit.

There is therefore provided a frequency generator for generating asignal at a desired frequency, with advantageous characteristics.

What is claimed is:
 1. A clock generator, for generating an output clocksignal, the clock generator comprising: a first clock signal input, forreceiving a first input clock signal; a first frequency comparator, forgenerating a first frequency comparison signal based on a ratio of afrequency of the output clock signal to a frequency of the first inputclock signal; a first subtractor, for forming a first error signalrepresenting a difference between an input desired frequency ratio andthe first frequency comparison signal; a first digital filter, forreceiving the first error signal and forming a filtered first errorsignal; a second clock signal input, for receiving a second input clocksignal; a second frequency comparator, for generating a second frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the second input clock signal; a secondsubtractor, for forming a second error signal representing a differencebetween the filtered first error signal and the second frequencycomparison signal; a second digital filter, for receiving the seconderror signal and forming a filtered second error signal; and anumerically controlled oscillator, for receiving the filtered seconderror signal and generating the output clock signal.
 2. A clockgenerator as claimed in claim 1, wherein the first digital filter is alow pass filter.
 3. A clock generator as claimed in claim 1, wherein thefirst digital filter is a first order integrator.
 4. A clock generatoras claimed in claim 1, wherein the second digital filter is a low passfilter.
 5. A clock generator as claimed in claim 4, wherein the seconddigital filter is a first order integrator.
 6. A clock generator asclaimed in claim 1, wherein the first digital filter is selected fromthe group comprising a low pass filter and a first order integrator,wherein the second digital filter is selected from the group comprisinga low pass filter and a first order integrator, and second digitalfilter has a wider bandwidth than the first digital filter.
 7. A clockgenerator as claimed in claim 1, wherein the first frequency comparatorcomprises a first counter, for counting a number of periods of theoutput clock signal per period of the first input clock signal.
 8. Aclock generator as claimed in claim 1, wherein the second frequencycomparator comprises a second counter, for counting a number of periodsof the output clock signal per period of the second input clock signal.9. A clock generator as claimed in claim 1, wherein the numericallycontrolled oscillator comprises a digital-analog converter, an output ofwhich controls the frequency of the output clock signal.
 10. A clockgenerator as claimed in claim 1, wherein the numerically controlledoscillator comprises a set of circuit elements, said set of circuitelements being digitally programmable such that a multi-bit binarysignal representing or derived from a numerical input can control whichof the circuit elements are connected, and can thus control a frequencyof the oscillator.
 11. A clock generator as claimed in claim 1, whereinthe numerically controlled oscillator comprises a word length reductioncircuit, for receiving the filtered second error signal and generating aword length reduced version thereof for use in controlling the frequencyof the output clock signal.
 12. A clock generator as claimed in claim 1,wherein the second frequency comparator further comprises: an input, forreceiving an estimated value of the ratio of the frequency of the outputclock signal to the frequency of the second input clock signal; and asubtractor, for subtracting the estimated value of the ratio of thefrequency of the output clock signal to the frequency of the secondinput clock signal from an actual value of the ratio of the frequency ofthe output clock signal to the frequency of the second input clocksignal, to generate the second frequency comparison signal.
 13. A clockgenerator as claimed in claim 1, wherein an output of the first digitalfilter is preset to an estimated value of the ratio of the frequency ofthe output clock signal to the frequency of the second input clocksignal on power-up of the clock generator.
 14. A clock generator asclaimed in claim 8, wherein the second counter is set to a count valueequal to a negative of an estimated value of the ratio of the frequencyof the output clock signal to the frequency of the second input clocksignal in each period of the second input clock signal.
 15. A clockgenerator as claimed in claim 1, comprising a first frequency divider,connected to divide the frequency of the output clock signal beforepassing to the first frequency comparator.
 16. A clock generator asclaimed in claim 1, comprising a second frequency divider, connected todivide the frequency of the output clock signal before passing to thesecond frequency comparator.
 17. A clock generator as claimed in claim1, comprising a first limiter, for limiting a value of the first errorsignal passed to the first low pass filter.
 18. A clock generator asclaimed in claim 17, comprising circuitry for detecting the absence of afirst clock input signal and activating said limiter in responsethereto.
 19. A clock generator as claimed in claim 1, comprising asecond limiter, for limiting a value of the second error signal passedto the second low pass filter.
 20. A clock generator as claimed in claim19, comprising circuitry for detecting the absence of a second clockinput signal and activating said limiter in response thereto.
 21. Aclock generator as claimed in claim 1, further comprising a modulator,for forming a sequence of input desired frequency ratio values having anaverage value equal to a desired frequency ratio.
 22. A clock generatoras claimed in claim 21, wherein the modulator is adapted to receivefirst and second modulator input values, and to form the sequence ofinput desired frequency ratio values having an average value equal to aratio of the first modulator input value to the second modulator inputvalue.
 23. A clock generator as claimed in claim 22, wherein themodulator is configurable to operate with second modulator input valuesthat can be represented as binary words with more than one non-zero bit.24. A clock generator as claimed in claim 21, wherein the modulator is adelta-sigma modulator.
 25. An audio processing integrated circuit,comprising: at least one digital audio interface, for receiving digitalaudio data with an accompanying audio data clock; a digital-analogconverter, for reconstructing analog audio data based on receiveddigital audio data; and a clock generator for generating an output clocksignal, the clock generator comprising: a first clock signal input, forreceiving said audio data clock as a first input clock signal; a firstfrequency comparator, for generating a first frequency comparison signalbased on a ratio of a frequency of the output clock signal to afrequency of the first input clock signal; a first subtractor, forforming a first error signal representing a difference between an inputdesired frequency ratio and the first frequency comparison signal; afirst digital filter, for receiving the first error signal and forming afiltered first error signal; a second clock signal input, for receivinga second input clock signal; a second frequency comparator, forgenerating a second frequency comparison signal based on a ratio of afrequency of the output clock signal to a frequency of the second inputclock signal; a second subtractor, for forming a second error signalrepresenting a difference between the filtered first error signal andthe second frequency comparison signal; a second digital filter, forreceiving the second error signal and forming a filtered second errorsignal; and a numerically controlled oscillator, for receiving thefiltered second error signal and generating the output clock signal,wherein the output clock signal of the clock generator is used as aclock for the digital-analog converter.
 26. An audio processingintegrated circuit as claimed in claim 25, further comprising: at leastone analog audio interface, for receiving analog audio data; ananalog-digital converter, for sampling the received analog audio data;and a clock generator as claimed in any preceding claim, wherein theoutput clock signal of the clock generator is used as the sampling clockfor the analog-digital converter.
 27. An audio processing integratedcircuit, as claimed in claim 25, comprising a plurality of digital audiointerfaces, and means for selecting between a corresponding plurality ofaudio data clocks as the first input clock signal to the clockgenerator.
 28. An audio processing integrated circuit, as claimed inclaim 27, configurable so that the clock, generated from an audio dataclock associated with one digital audio interface that is selected asthe first input clock signal of the clock generator, is used to clockanother digital audio interface.
 29. A device containing an audioprocessing integrated circuit, wherein the audio processing integratedcircuit comprises: at least one digital audio interface, for receivingdigital audio data with an accompanying audio data clock; adigital-analog converter, for reconstructing analog audio data based onreceived digital audio data; and a clock generator for generating anoutput clock signal, the clock generator comprising: a first clocksignal input, for receiving said audio data clock as a first input clocksignal; a first frequency comparator, for generating a first frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the first input clock signal; a firstsubtractor, for forming a first error signal representing a differencebetween an input desired frequency ratio and the first frequencycomparison signal; a first digital filter, for receiving the first errorsignal and forming a filtered first error signal; a second clock signalinput, for receiving a second input clock signal; a second frequencycomparator, for generating a second frequency comparison signal based ona ratio of a frequency of the output clock signal to a frequency of thesecond input clock signal; a second subtractor, for forming a seconderror signal representing a difference between the filtered first errorsignal and the second frequency comparison signal; a second digitalfilter, for receiving the second error signal and forming a filteredsecond error signal; and a numerically controlled oscillator, forreceiving the filtered second error signal and generating the outputclock signal, wherein the output clock signal of the clock generator isused as a clock for the digital-analog converter, the device furthercomprising: a communications processor for receiving cellular calls, theprocessor being coupled to the at least one digital audio interface viaa digital audio bus.
 30. A device containing an audio processingintegrated circuit, wherein the audio processing integrated circuitcomprises: at least one digital audio interface, for receiving digitalaudio data with an accompanying audio data clock; a digital-analogconverter, for reconstructing analog audio data based on receiveddigital audio data; and a clock generator for generating an output clocksignal, the clock generator comprising: a first clock signal input, forreceiving said audio data clock as a first input clock signal; a firstfrequency comparator, for generating a first frequency comparison signalbased on a ratio of a frequency of the output clock signal to afrequency of the first input clock signal; a first subtractor, forforming a first error signal representing a difference between an inputdesired frequency ratio and the first frequency comparison signal; afirst digital filter, for receiving the first error signal and forming afiltered first error signal; a second clock signal input, for receivinga second input clock signal; a second frequency comparator, forgenerating a second frequency comparison signal based on a ratio of afrequency of the output clock signal to a frequency of the second inputclock signal; a second subtractor, for forming a second error signalrepresenting a difference between the filtered first error signal andthe second frequency comparison signal; a second digital filter, forreceiving the second error signal and forming a filtered second errorsignal; and a numerically controlled oscillator, for receiving thefiltered second error signal and generating the output clock signal,wherein the output clock signal of the clock generator is used as aclock for the digital-analog converter, the device further comprising:an applications processor for retrieving audio data from local memory,the applications processor being coupled to the at least one digitalaudio interface via a digital audio bus.
 31. A device containing anaudio processing integrated circuit, wherein the audio processingintegrated circuit comprises: at least one digital audio interface, forreceiving digital audio data with an accompanying audio data clock; adigital-analog converter, for reconstructing analog audio data based onreceived digital audio data; and a clock generator for generating anoutput clock signal, the clock generator comprising: a first clocksignal input, for receiving said audio data clock as a first input clocksignal; a first frequency comparator, for generating a first frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the first input clock signal; a firstsubtractor, for forming a first error signal representing a differencebetween an input desired frequency ratio and the first frequencycomparison signal; a first digital filter, for receiving the first errorsignal and forming a filtered first error signal; a second clock signalinput, for receiving a second input clock signal; a second frequencycomparator, for generating a second frequency comparison signal based ona ratio of a frequency of the output clock signal to a frequency of thesecond input clock signal; a second subtractor, for forming a seconderror signal representing a difference between the filtered first errorsignal and the second frequency comparison signal; a second digitalfilter, for receiving the second error signal and forming a filteredsecond error signal; and a numerically controlled oscillator, forreceiving the filtered second error signal and generating the outputclock signal, wherein the output clock signal of the clock generator isused as a clock for the digital-analog converter, the device furthercomprising: a wireless modem for transmitting and/or receiving audiodata from a peripheral device, the modem being coupled to the at leastone digital audio interface via a digital audio bus.
 32. A devicecontaining an audio processing integrated circuit, wherein the audioprocessing integrated circuit comprises: a plurality of digital audiointerfaces, for receiving digital audio data with respectiveaccompanying audio data clocks; means for selecting between theplurality of audio data clocks as a first input clock signal to a clockgenerator for generating an output clock signal, the clock generatorcomprising: a first clock signal input, for receiving said selectedaudio data clock as the first input clock signal; a first frequencycomparator, for generating a first frequency comparison signal based ona ratio of a frequency of the output clock signal to a frequency of thefirst input clock signal; a first subtractor, for forming a first errorsignal representing a difference between an input desired frequencyratio and the first frequency comparison signal; a first digital filter,for receiving the first error signal and forming a filtered first errorsignal; a second clock signal input, for receiving a second input clocksignal; a second frequency comparator, for generating a second frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the second input clock signal; a secondsubtractor, for forming a second error signal representing a differencebetween the filtered first error signal and the second frequencycomparison signal; a second digital filter, for receiving the seconderror signal and forming a filtered second error signal; and anumerically controlled oscillator, for receiving the filtered seconderror signal and generating the output clock signal, wherein the outputclock signal of the clock generator is used as a clock for adigital-analog converter, for reconstructing analog audio data based onreceived digital audio data; and the device further comprising: acommunications processor for receiving cellular calls, thecommunications processor being coupled to a first digital audiointerface via a first digital audio bus; and an applications processorfor retrieving audio data from local memory, the applications processorbeing coupled to a second digital audio interface via a second digitalaudio bus.
 33. A device as claimed in claim 32, the device beingconfigurable so that the audio data clock that is selected as the firstinput clock signal of the clock generator is associated with the firstdigital audio interface.
 34. A device as claimed in claim 33, whereinthe first input clock signal is used to clock the second digital audiointerface.
 35. A device containing an audio processing integratedcircuit, wherein the audio processing integrated circuit comprises: aplurality of digital audio interfaces, for receiving digital audio datawith respective accompanying audio data clocks; means for selectingbetween the plurality of audio data clocks as a first input clock signalto a clock generator for generating an output clock signal, the clockgenerator comprising: a first clock signal input, for receiving saidselected audio data clock as the first input clock signal; a firstfrequency comparator, for generating a first frequency comparison signalbased on a ratio of a frequency of the output clock signal to afrequency of the first input clock signal; a first subtractor, forforming a first error signal representing a difference between an inputdesired frequency ratio and the first frequency comparison signal; afirst digital filter, for receiving the first error signal and forming afiltered first error signal; a second clock signal input, for receivinga second input clock signal; a second frequency comparator, forgenerating a second frequency comparison signal based on a ratio of afrequency of the output clock signal to a frequency of the second inputclock signal; a second subtractor, for forming a second error signalrepresenting a difference between the filtered first error signal andthe second frequency comparison signal; a second digital filter, forreceiving the second error signal and forming a filtered second errorsignal; and a numerically controlled oscillator, for receiving thefiltered second error signal and generating the output clock signal,wherein the output clock signal of the clock generator is used as aclock for a digital-analog converter, for reconstructing analog audiodata based on received digital audio data; 20 and the device furthercomprising: a communications processor for receiving cellular calls, thecommunications processor being coupled to a first digital audiointerface via a first digital audio bus; an applications processor forretrieving audio data from local memory, the applications processorbeing coupled to a second digital audio interface via a second digitalaudio bus; and a wireless modem for transmitting and/or receiving audiodata from a peripheral device, the modem being coupled to a thirddigital audio interface via a third digital audio bus.
 36. A device asclaimed in claim 35, the device being configurable so that the audiodata clock that is selected as the first input clock signal of the clockgenerator is associated with the first digital audio interface.
 37. Adevice as claimed in claim 36 wherein the first input clock signal isused to clock the second digital audio interface.
 38. A devicecomprising an audio processing integrated circuit, the audio processingintegrated circuit comprising: at least one digital audio interface, forreceiving digital audio data with an accompanying audio data clock; adigital-analog converter, for reconstructing analog audio data based onreceived digital audio data; and a clock generator for generating anoutput clock signal, the clock generator comprising: a first clocksignal input, for receiving said audio data clock as a first input clocksignal; a first frequency comparator, for generating a first frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the first input clock signal; a firstsubtractor, for forming a first error signal representing a differencebetween an input desired frequency ratio and the first frequencycomparison signal; a first digital filter, for receiving the first errorsignal and forming a filtered first error signal; a second clock signalinput, for receiving a second input clock signal; a second frequencycomparator, for generating a second frequency comparison signal based ona ratio of a frequency of the output clock signal to a frequency of thesecond input clock signal; a second subtractor, for forming a seconderror signal representing a difference between the filtered first errorsignal and the second frequency comparison signal; a second digitalfilter, for receiving the second error signal and forming a filteredsecond error signal; and a numerically controlled oscillator, forreceiving the filtered second error signal and generating the outputclock signal, wherein the output clock signal of the clock generator isused as a clock for the digital-analog converter, wherein the receiveddigital audio data and accompanying audio data clock are received by thedevice in burst mode, and the digital-analog converter is continuouslysupplied with the output clock signal of the clock generator, and theoutput clock signal of the clock generator is based on the burst modeaccompanying audio data clock.
 39. A device as claimed in claim 38,wherein the received burst mode digital audio data and accompanyingaudio data clock are received from a USB source.
 40. A device,comprising one of: a portable electronic device, a mobile phone, a PDA,a netbook computer, a laptop computer, a tablet computer, a computer, asmartphone, a digital camera or an audio convergence device, containingan audio processing integrated circuit, wherein the audio processingintegrated circuit comprises: at least one digital audio interface, forreceiving digital audio data with an accompanying audio data clock; adigital-analog converter, for reconstructing analog audio data based onreceived digital audio data; and a clock generator for generating anoutput clock signal, the clock generator comprising: a first clocksignal input, for receiving said audio data clock as a first input clocksignal; a first frequency comparator, for generating a first frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the first input clock signal; a firstsubtractor, for forming a first error signal representing a differencebetween an input desired frequency ratio and the first frequencycomparison signal; a first digital filter, for receiving the first errorsignal and forming a filtered first error signal; a second clock signalinput, for receiving a second input clock signal; a second frequencycomparator, for generating a second frequency comparison signal based ona ratio of a frequency of the output clock signal to a frequency of thesecond input clock signal; a second subtractor, for forming a seconderror signal representing a difference between the filtered first errorsignal and the second frequency comparison signal; a second digitalfilter, for receiving the second error signal and forming a filteredsecond error signal; and a numerically controlled oscillator, forreceiving the filtered second error signal and generating the outputclock signal, wherein the output clock signal of the clock generator isused as a clock for the digital-analog converter.
 41. An integratedcircuit, comprising: a clock generator, for generating an output clocksignal, the clock generator comprising: a first clock signal input, forreceiving a first input clock signal; a first frequency comparator, forgenerating a first frequency comparison signal based on a ratio of afrequency of the output clock signal to a frequency of the first inputclock signal; a first subtractor, for forming a first error signalrepresenting a difference between an input desired frequency ratio andthe first frequency comparison signal; a first digital filter, forreceiving the first error signal and forming a filtered first errorsignal; a second clock signal input, for receiving a second input clocksignal; a second frequency comparator, for generating a second frequencycomparison signal based on a ratio of a frequency of the output clocksignal to a frequency of the second input clock signal; a secondsubtractor, for forming a second error signal representing a differencebetween the filtered first error signal and the second frequencycomparison signal; a second digital filter, for receiving the seconderror signal and forming a filtered second error signal; and anumerically controlled oscillator, for receiving the filtered seconderror signal and generating the output clock signal, and furthercomprising: a first oscillator, for generating a first oscillator clocksignal, and being connected to the first clock signal input of the clockgenerator; and a second oscillator, for generating a second oscillatorclock signal, and being connected to the second clock signal input ofthe clock generator, wherein the first and second oscillators areconfigured such that the first oscillator clock signal has lower jitterthan the second oscillator clock signal, and the second clock signal hashigher frequency accuracy than the first oscillator clock signal.
 42. Anintegrated circuit as claimed in claim 41, wherein the first oscillatoris an RC-25 oscillator.
 43. An integrated circuit as claimed in claim42, wherein the RC-oscillator comprises a temperature stable capacitanceand a zero-temperature-coefficient resistance.
 44. An integrated circuitas claimed in claim 41, wherein one or more component of the firstoscillator may be trimmed to obtain a desired frequency of the firstoscillator clock signal.
 45. An integrated circuit as claimed in claim41, wherein the second oscillator is an LC-type oscillator.
 46. Anintegrated circuit as claimed in claim 41, wherein the second oscillatoris a MEMS (Micro-Electro-Mechanical-System) type oscillator.